The present invention relates to methods for heat treating ferrous material and, more particularly, to methods for annealing tool steels under nitrogen based atmospheres.
In annealing ferrous material, such as for purposes of stress relief, etc., it is important to avoid changes in the surface metallurgy of the material. Thus, surface carburization and decarburization are to be avoided during annealing. It has been found that most of the tool steels have a strong tendency to decarburize during annealing even at relatively low temperatures. In addition, it has been common practice to utilize protective atmospheres such as endothermic, exothermic, or others, in the hot zones of annealing furnaces. However, as these atmospheres are derived from hydrocarbon sources such as natural gas (i.e. methane), the cost of producing these atmospheres has increased significantly in recent years. Furthermore, expensive generator devices are necessary to produce these atmospheres and these devices require considerable maintenance but yet are relatively inflexible in that they are not effective to produce a wide range of atmospheres over a wide range of flow rates. Typically, the foregoing conventional generated atmospheres are enriched with natural gas so that an adequate level of hydrocarbon is available to react with oxidants leaking to the furnace and thereby avoid decarburization or oxidation of the ferrous material being annealed.
In order to avoid carburization or decarburization of ferrous material at a particular temperature in a furnace hot zone, it is necessary to maintain an equilibrium condition between CO.sub.2 and CO, i.e. maintain a predetermined ratio at a given temperature. Similarly, a given ratio between H.sub.2 O and H.sub.2 must be maintained to avoid oxidation of ferrous material at a particular temperature. A discussion of these ratios appears in Wire Technology, November/December, 1979, pages 51-57 which indicates that by adding a hydrocarbon to a nitrogen based atmosphere, the level of CO increases by virtue of the reaction: EQU CH.sub.4 +CO.sub.2 .revreaction.2CO+2H.sub.2
and such an increase in CO tends to reduce the ratio of CO.sub.2 /CO at constant temperature to thereby reduce the decarburizing tendency of the atmosphere. Thus, methane addition is effective to control or limit the tendency of a furnace atmosphere to decarburize, but when annealing at lower temperatures, i.e. below about 1450.degree. F., methane is relatively sluggish and simply fails to react sufficiently to avoid decarburization. Continued additions of methane fail to cure this problem.
The technique of introducing nitrogen based atmospheres comprised of nitrogen and propane into a furnace hot zone is described in Iron and Steel Engineer, November, 1980, pages 51-57. This article notes at page 52 that propane is relatively difficult to add precisely and insufficient propane results in decarburization while excessive propane leads to sooting, both highly undesirable results. This article suggests that preferred annealing atmospheres can be developed from mixtures of nitrogen and methanol (CH.sub.3 OH) being introduced into a furnace hot zone. Methanol will dissociate into CO and H.sub.2 and will alter the ratios of CO.sub.2 /CO and H.sub.2 O/H.sub.2 such that tendencies toward decarburization will be reduced. Those skilled in the art will appreciate that use of methanol is costly in that separate storage vessels, pumping equipment and piping, etc. are required in order to store and transfer methanol to a heat treating furnace. Consequently, use of methanol is an expensive and rather complicated approach toward generating protective atmospheres in heat treating furnaces.
Consequently, a clear need exists for processes for annealing ferrous material such as tool steel in acceptable "soak" time periods under a nitrogen based atmosphere such that decarburization and sooting are avoided but which is not inordinately expensive and does not necessarily require methanol addition to the furnace atmosphere.